(171e) Linking Molecular Conformation to Charge Transport in Organic Materials

Solution-processed conjugated polymers are an important class of semiconductor, largely due to the ability to modify their optical and electronic properties through molecular design choices and their compatibility with low-cost, scalable manufacturing processes. An important property of these materials is the charge-carrier mobility, which often determines the performance of organic electronic devices, and depends sensitively on the active layer morphology. In order to manufacture the most efficient devices, it is therefore vital to optimize the morphology such that the charge-carrier mobility is maximized. However, understanding the relationship between morphology on the molecular level and the subsequent device-scale performance is a significant challenge spanning multiple length and time-scales.

We use coarse-grained molecular dynamics simulations to simulate a variety of pristine morphologies of thiophene-based conjugated oligomers with different side-chain architectures and under various annealing protocols. The range of morphologies considered allows the analysis of several ordered and disordered structures, including hexagonally packed cylinders, ribbons and Ï?-stacked lamellar structures, depending on the processing conditions. The charge transport characteristics are then determined for these morphologies using a combination of atomistic backmapping, efficient semi-empirical quantum chemical calculations that bypass the requirement for density functional theory, and kinetic Monte Carlo simulations to elucidate the hole mobility dependence on side-chain architecture and annealing temperature. Percolation analysis is also used to identify the structures that allow for the most efficient charge transport through the device. This demonstrates a highly-flexible, modular pipeline that can be used to connect sub nano-scale morphology to bulk thin-film behavior, informing future charge transport investigations and manufacturing processes alike.

Our data echoes experimentally obtained trends concerning the hole mobility, namely that increased annealing temperature and regioregular side-chain placement improves charge transport. Using our methodology, we quantify this behavior and attribute these trends to regular side-chain architectures permitting the assembly into closely-packed Ï?-stacking crystals, amplifying the rate of inter-chain hopping, which is the limiting factor for efficient charge transport.